title: The L3 loop: a structural motif determining specific interactions between SMAD proteins and TGF-beta receptors.
abstract: Signal transduction specificity in the transforming growth factor-beta (TGF-beta) system is determined by ligand activation of a receptor complex which then recruits and phosphorylates a subset of SMAD proteins including Smads 1 and 2. These then associate with Smad4 and move into the nucleus where they regulate transcription. We have identified a discrete surface structure in Smads 1 and 2 that mediates and specifies their receptor interactions. This structure is the L3 loop, a 17 amino acid region that protrudes from the core of the conserved SMAD C-terminal domain. The L3 loop sequence is invariant among TGF-beta- and bone morphogenetic protein (BMP)-activated SMADS, but differs at two positions between these two groups. Swapping these two amino acids in Smads 1 and 2 induces a gain or loss, respectively, in their ability to associate with the TGF-beta receptor complex and causes a switch in the phosphorylation of Smads 1 and 2 by the BMP and TGF-beta receptors, respectively. A full switch in phosphorylation and activation of Smads 1 and 2 is obtained by swapping both these two amino acids and four amino acids near the C-terminal receptor phosphorylation sites. These studies identify the L3 loop as a determinant of specific SMAD-receptor interactions, and indicate that the L3 loop, together with the C-terminal tail, specifies SMAD activation.
authors.affiliation: Cell Biology Program, Howard Hughes Medical Institute, Memorial Sloan-Kettering Cancer Center, New York 10021, USA.
authors.forename: R S
authors.initials: RS
authors.lastname: Lo
authors.forename: Y G
authors.initials: YG
authors.lastname: Chen
authors.forename: Y
authors.initials: Y
authors.lastname: Shi
authors.forename: N P
authors.initials: NP
authors.lastname: Pavletich
authors.forename: J
authors.initials: J
authors.lastname: Massagué
mesh_terms: D000595:Amino Acid Sequence; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism*; D012097:Repressor Proteins; D015398:Signal Transduction; D051899:Smad2 Protein; D013329:Structure-Activity Relationship; D015534:Trans-Activators
chemical_list: D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins; C079439:MXD1 protein, human; D018125:Receptors, Transforming Growth Factor beta; D012097:Repressor Proteins; C497782:SMAD2 protein, human; D051899:Smad2 Protein; D015534:Trans-Activators